The invention relates to one-part silylated polymer adhesive/sealant compositions, and more particularly to the use of a hydrophilic solvent to improve the cure-through rate of such adhesive/sealant compositions.
Compositions containing vulcanizable silicon terminated organic polymers are well known in the art, and are commonly used as adhesives and/or sealants. For example, U.S. Pat. No. 3,632,557, incorporated herein by reference, describes room temperature curable silicon terminated organic polymers made by the reaction of an isocyanate terminated polyurethane prepolymer having urethane linkages and an organic silicon compound. Such polymers are commonly referred to as silylated polymers, and find use as adhesives and sealants, particularly in the automobile industry. Other patents relating to silylated polymers include U.S. Pat. Nos. 3,979,344; 4,222,925; 4,889,903; 4,894,426; 4,954,598 and 5,097,053, all incorporated herein by reference.
It is often desirable to provide such silylated polymer adhesive/sealant compositions in packages suitable for direct application of the product. A popular, efficient and economical type of package is an aluminum or aluminum-lined cartridge. Unfortunately, it has been discovered that such adhesive/sealant compositions often react with the aluminum, and corrode through the sides of such cartridges. Once the composition corrodes through the cartridge and is exposed to moisture in air, the curing process begins, and the adhesive/sealant solidifies before it can be used.
In using such adhesives and/or sealants, there are two important cure rates which need to be considered. The first is the surface cure rate, generally expressed as the tack-free time, that is, the time until the surface cures to the point of not being tacky. Generally, the application of the composition must be completed before such surface curing is complete. Therefore, the shorter the tack-free time, the less time one has to work with the adhesive/sealant. The second important cure rate is the cure-through rate. This is a measure of the time which it takes for the composition to cure through its thickness. The shorter the cure-through time, the sooner the finished product can be used. Therefore, it is often desirable to speed up the cure through rate of an adhesive/sealant while maintaining or even increasing the tack free time.
The present inventors made the discovery that methanol, which is commonly used in formulations of such adhesive/sealant compositions, reacts with aluminum when in the presence of the catalysts which normally are also included in such compositions. In an attempt to overcome the corrosion problem, the present inventors tested compositions prepared without methanol. Such compositions were found to be non-reactive with aluminum, and therefore to have a much longer shelf life in aluminum cartridges. Unfortunately, it was also found that the cure-through rate of silylated polymer adhesives and/or sealants without methanol was significantly slower than that of such adhesives/sealants with methanol. Increasing the amount of catalysts and using different catalyst systems did not improve the cure-through rate. In some cases, adding additional catalyst or using alternate catalysts was found to decrease the tack-free time significantly, thus making the adhesive/sealant more difficult to handle, but without improving the cure-through rate. Thus, there is a need for a methanol-free silylated polymer adhesive/sealant composition which is non-corrosive to aluminum with at least as fast a cure-through rate as compositions containing methanol.
In accordance with the present invention, it was discovered that the addition of a small amount of one or more hydrophilic solvents to a one-part silylated polymer based adhesive/sealant composition speeds up the cure-through rate of such compositions. It was further discovered that the elimination of methanol solves the corrosion problem when such compositions are packaged in aluminum containers. Therefore, the present invention provides a silylated polymer-based adhesive/sealant composition comprising a small but effective amount of one or more hydrophilic solvents, which composition is substantially free of methanol and other alcohols. Adhesives and/or sealants made in accordance with the present invention are non-reactive with aluminum and have a faster cure-through rate than such compositions without the solvent. For purposes of this application, an effective amount of such a solvent is that amount which speeds up the cure-though rate of an adhesive/sealant composition as compared to the same adhesive/sealant composition without the solvent. A suitable method for determining the cure-through rate is set forth below.
In addition to avoiding methanol, it is believed that alcohols other than methanol, such as ethanol or propanol, should also be avoided in such compositions. Such alcohols may undesirably react with other components, particularly the silane components, thus adversely affecting the properties or performance of the adhesive/sealant compositions. Such reactions may also result in the formation of methanol as a byproduct. Therefore, the compositions of the present invention desirably are free of methanol and any other alcohols as well.
Another embodiment of the invention is a method of accelerating the cure-through rate of a one-part, moisture-curable silylated polymer-based adhesive/sealant composition comprising adding an effective amount of an alcohol-free hydrophilic solvent to the composition.
A further embodiment of the present invention is a packaged adhesive/sealant composition comprising:
a) an aluminum or aluminum-lined package; and
b) a one-part, moisture-curable silylated polymer-based adhesive/sealant composition comprising a moisture-curable silylated polymer and an effective amount of a hydrophilic solvent,
wherein the composition is sealed within the package, is substantially free of alcohols and is non-reactive with the aluminum of the package.
Although applicants do not wish to be bound by a particular theory of how the invention works, it is believed that when the hydrophilic solvent evaporates from the adhesive/sealant composition it creates channels for moisture to get into the adhesive/sealant and thereby cure the silylated polymer. If there is no hydrophilic solvent in the composition, then the moisture has more difficulty reaching the interior of the uncured material. As a result, although the tack-free time remains the same or even increases, the cure-through time is significantly diminished.
Hydrophilic solvents are those which have an affinity with water. However, because the silylated polymer compositions are cured by exposure to water, the solvents should be as anhydrous as is reasonably possible. Suitable hydrophilic solvents for use in the present invention should be highly volatile, and include those oxygen and/or nitrogen containing organic solvents whose flash points are less than about 45xc2x0 C. (open cup) and whose evaporation rates are at least 1, with n-butyl acetate defined as having an evaporation rate of 1. However, alcohols should not be used in the solvent because of possible undesirable reactions with aluminum in the packaging or with other components of the composition. Suitable hydrophilic solvents include substituted or unsubstituted esters, ketones and amines, provided they are sufficiently volatile and non-reactive with the other components of the compositions. Among the hydrophilic solvents suitable for use in the present invention are esters such as amyl formate, ethyl acetate, isopropyl acetate, n-propyl acetate, ethyl propionate, isobutyl acetate and n-butyl acetate; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone and methyl isobutyl ketone; amines such as isopropylamine, sec-butylamine, n-amylamine, sec-hexylamine, ethylamine, ethylene diamine, propyl imine, acetonitrile, n-butyronitrile and morpholine; mixture of such solvents and mixtures of them with other anhydrous solvents, especially their azeotropes.
The small amount of hydrophilic solvent used in the composition is not critical, but preferably should be as small as possible. Preferably no more than about 10 percent by weight (wt %) is used, more preferably less than about 6 wt %, and most preferably less than about 3 wt %. To obtain the desired cure-through rate acceleration, preferably a minimum of about 0.1 wt % is used, more preferably at least about 0.5 wt %, and most preferably at least about 1 wt %. Thus, the amount of hydrophilic solvent used is desirably in the range of about 0.1 to about 10 wt %, preferably from about 0.5 to about 6 wt %, more preferably from about 1 to about 3 wt %. All percentages herein are by weight unless indicated otherwise.
The following test methods were employed in the examples set forth below. Tack-free time was measured in accordance with ASTM C679-87 (Reapproved 1992) entitled xe2x80x9cStandard Test Method for Tack-Free Time of Elastomeric Sealants,xe2x80x9d incorporated herein by reference. As indicated in the ASTM standard, the test consists of lightly touching a surface of a curing sealant with a polyethylene film at regular intervals until the sealant does not attach itself to the film and the film appears clean when peeled from the surface.
The cure-through rate of adhesive samples is measured by the following procedure. A sample of the test adhesive is prepared and conditioned to 23xc2x0 C. for at least 6 hours. A polyethylene container is provided which is as least 45 mm in diameter and 10 mm in height, and purged with dry nitrogen. The container is filled with the test adhesive, and the top surface leveled with a spatula. The sample is placed in a 23xc2x0 C., 50% relative humidity (RH) chamber, in which it cures from the top surface down. After 24 hours, the central thickness of the cured disk-like top portion of the sample is measured.
The materials used in the following examples are all commercially available and/or may be prepared by methods well known in the art, as discussed, for example, in some of the references incorporated by reference above.